Illumination system of a microlithographic projection exposure apparatus

ABSTRACT

An illumination system includes an optical integrator having a plurality of light entrance facets, whose images at least substantially superimpose in a mask plane. A spatial light modulator transmits or reflects impinging projection light in a spatially resolved manner. A pupil forming unit directs projection light onto the spatial light modulator. An objective images a light exit surface of the spatial light modulator onto the light entrance facets of the optical integrator so that an image of an object area on the light exit surface completely coincides with one of the light entrance facets. A control unit controls the spatial light modulator such that along a scan direction a length of an image, which is formed on a mask from a light pattern in the object area, gradually increases at a beginning of a scan cycle and gradually decreases at the end of the scan cycle.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims benefit under 35 USC 119 of EuropeanApplication Nos. 13194135.3, filed Nov. 22, 2013 and 14155685.2, filedFeb. 19, 2014. The contents of both of these applications are herebyincorporated by reference in its entirety.

FIELD

The disclosure generally relates to illumination systems forilluminating a mask in a microlithographic exposure apparatus, and inparticular to such systems including an optical integrator configured toproduce a plurality of secondary light sources in a pupil plane. Thedisclosure also relates to a method of operating such illuminationsystems.

BACKGROUND

Microlithography (also referred to as photolithography or simplylithography) is a technology for the fabrication of integrated circuits,liquid crystal displays and other microstructured devices. The processof microlithography, in conjunction with the process of etching, is usedto pattern features in thin film stacks that have been formed on asubstrate, for example a silicon wafer. At each layer of thefabrication, the wafer is first coated with a photoresist which is amaterial that is sensitive to radiation, such as deep ultraviolet (DUV)or vacuum ultraviolet (VUV) light. Next, the wafer with the photoresiston top is exposed to projection light in a projection exposureapparatus. The apparatus projects a mask containing a pattern onto thephotoresist so that the latter is only exposed at certain locationswhich are determined by the mask pattern. After the exposure thephotoresist is developed to produce an image corresponding to the maskpattern. Then an etch process transfers the pattern into the thin filmstacks on the wafer. Finally, the photoresist is removed. Repetition ofthis process with different masks results in a multi-layeredmicrostructured component.

A projection exposure apparatus typically includes a light source, anillumination system that illuminates the mask with projection lightproduced by the light source, a mask stage for aligning the mask, aprojection objective and a wafer alignment stage for aligning the wafercoated with the photoresist. The illumination system illuminates a fieldon the mask that may have the shape of a rectangular or curved slit, forexample.

In current projection exposure apparatus a distinction can be madebetween two different types of apparatus. In one type each targetportion on the wafer is irradiated by exposing the entire mask patternonto the target portion in one go. Such an apparatus is commonlyreferred to as a wafer stepper. In the other type of apparatus, which iscommonly referred to as a step-and-scan apparatus or scanner, eachtarget portion is irradiated by progressively scanning the mask patternunder the projection beam along a scan direction while synchronouslymoving the substrate parallel or anti-parallel to this direction. Theratio of the velocity of the wafer and the velocity of the mask is equalto the magnification of the projection objective, which is usuallysmaller than 1, for example 1:4.

It is to be understood that the term “mask” (or reticle) is to beinterpreted broadly as a patterning mechanism. Commonly used maskscontain opaque or reflective patterns and may be of the binary,alternating phase-shift, attenuated phase-shift or various hybrid masktype, for example. However, there are also active masks, e.g. masksrealized as a programmable mirror array. Also programmable LCD arraysmay be used as active masks.

As the technology for manufacturing microstructured devices advances,there are ever increasing demands also on the illumination system.Ideally, the illumination system illuminates each point of theillumination field on the mask with projection light having a welldefined spatial and angular irradiance distribution. The term angularirradiance distribution describes how the total light energy of a lightbundle, which converges towards a particular point in the mask plane, isdistributed among the various directions of the rays that constitute thelight bundle.

The angular irradiance distribution of the projection light impinging onthe mask is usually adapted to the kind of pattern to be projected ontothe photoresist. Often the angular irradiance distribution depends onthe size, orientation and pitch of the features contained in thepattern. The most commonly used angular irradiance distributions ofprojection light are referred to as conventional, annular, dipole andquadrupole illumination settings. These terms refer to the irradiancedistribution in a pupil plane of the illumination system. With anannular illumination setting, for example, only an annular region isilluminated in the pupil plane. Thus there is only a small range ofangles present in the angular irradiance distribution of the projectionlight, and all light rays impinge obliquely with similar angles onto themask.

Different mechanisms are known in the art to modify the angularirradiance distribution of the projection light in the mask plane so asto achieve the desired illumination setting. In the simplest case a stop(diaphragm) including one or more apertures is positioned in a pupilplane of the illumination system. Since locations in a pupil planetranslate into angles in a Fourier related field plane such as the maskplane, the size, shape and location of the aperture(s) in the pupilplane determines the angular irradiance distributions in the mask plane.However, any change of the illumination setting involves a replacementof the stop. This makes it difficult to finely adjust the illuminationsetting, because this would involve a very large number of stops thathave aperture(s) with slightly different sizes, shapes or locations.Furthermore, the use of stops inevitably results in light losses andthus in a reduced throughput of the apparatus.

Many common illumination systems therefore include adjustable elementsthat make it possible, at least to a certain extent, to continuouslyvary the illumination of the pupil plane. Many illumination systems usean exchangeable diffractive optical element to produce a desired spatialirradiance distribution in the pupil plane. If zoom optics and a pair ofaxicon elements are provided between the diffractive optical element andthe pupil plane, it is possible to adjust this spatial irradiancedistribution.

Recently it has been proposed to use mirror arrays that illuminate thepupil plane. In EP 1 262 836 A1 the mirror array is realized as amicro-electromechanical system (MEMS) including more than 1000microscopic mirrors. Each of the mirrors can be tilted in two differentplanes perpendicular to each other. Thus radiation incident on such amirror device can be reflected into (substantially) any desireddirection of a hemisphere. A condenser lens arranged between the mirrorarray and the pupil plane translates the reflection angles produced bythe mirrors into locations in the pupil plane. This known illuminationsystem makes it possible to illuminate the pupil plane with a pluralityof spots, wherein each spot is associated with one particularmicroscopic mirror and is freely movable across the pupil plane bytilting this mirror.

Similar illumination systems are known from US 2006/0087634 A1, U.S.Pat. No. 7,061,582 B2, WO 2005/026843 A2 and WO 2010/006687 A1. US2010/0157269 A1 discloses an illumination system in which an array ofmicromirrors is directly imaged on the mask.

As mentioned further above, it is usually desired to illuminate, atleast after scan integration, all points on the mask with the sameirradiance and angular irradiance distribution. If points on the maskare illuminated with different irradiances, this usually results inundesired variations of the critical dimension (CD) on wafer level. Forexample, in the presence of irradiance variations the image of a linehaving a uniform width on the light sensitive surface may also haveirradiance variations along its length. Because of the fixed exposurethreshold of the resist, such irradiance variations directly translateinto widths variations of a structure that shall be defined by the imageof the line.

If the angular irradiance distribution varies over the illuminationfield on the mask, this also has a negative impact on the quality of theimage that is produced on the light sensitive surface. For example, ifthe angular irradiance distribution is not perfectly balanced, i.e morelight impinges from one side on a mask point than from the oppositeside, the conjugate image point on the light sensitive surface will belaterally shifted if the light sensitive surface is not perfectlyarranged in the focal plane of the projection objective. For modifyingthe spatial irradiance distribution in the illumination field U.S. Pat.No. 6,404,499 A and US 2006/0244941 A1 propose mechanical devices thatinclude two opposing arrays of opaque finger-like stop elements that arearranged side by side and aligned parallel to the scan direction. Eachpair of mutually opposing stop elements can be displaced along the scandirection so that the distance between the opposing ends of the stopelements is varied. If this device is arranged in a field plane of theillumination system that is imaged by an objective on the mask, it ispossible to produce a slit-shaped illumination field whose width alongthe scan direction may vary along the cross-scan direction. Since theirradiance is integrated during each scan cycle, the integratedirradiance (sometimes also referred to as illumination close) can befinely adjusted for a plurality of cross-scan positions in theillumination field.

However, these devices are mechanically very complex and expensive. Thisis also due to the fact that these devices have to be arranged in orvery close to a field plane in which usually the blades of a movablefield stop is arranged.

Adjusting the angular irradiance distribution in a field dependentmanner is more difficult. This is mainly because the spatial irradiancedistribution is only a function of the spatial coordinates x, y, whereasthe angular irradiance distribution also depends on the direction ofincidence given by a pair of angles α, β.

WO 2012/100791 A1 discloses an illumination system in which a firstmirror array is used to produce a desired irradiance distribution in thepupil plane of the illumination system. In close proximity to the pupilplane an optical integrator is arranged that has a plurality of lightentrance facets. Thus images of the light entrance facets aresuperimposed on the mask. The light spots produced by the mirror arrayhave an area that is at least five times smaller than the total area ofthe light entrance facets. Thus it is possible to produce variable lightpatterns on the light entrance facets. In this manner different angularirradiance distributions can be produced on different portions of theillumination field. It is thus possible, for example, to produce an Xdipole and a Y dipole illumination setting at a given time in theillumination field.

In order to ensure that the portions with different illuminationsettings are sharply delimited, it is proposed to use a second mirrorarray configured as a digital mirror device (DMD). This second mirrorarray is illuminated by the first mirror array and is imaged on thelight entrance facets by an objective. By bringing larger groups ofmicromirrors of the second mirror array in an “off”-state, it ispossible to produce irradiance distributions on the light entrancefacets that have sharp boundaries.

However, it turned out that it is difficult to produce so many and sosmall freely movable light spots with the first mirror array.Furthermore, this prior art illumination system is mainly concerned withproducing completely different illumination settings at differentportions in the illumination field. For that reason the light entrancefacets are usually not completely, but only partially illuminated.

In prior art illumination systems of the scanner type an adjustablefield stop including movable blades opens and closes the slit-likeillumination field at the beginning and the end of each scan cycle,respectively. This ensures that only the desired portions on the maskare projected on the light sensitive surface. The adjustable field stophas to perform its task reliably and with the highest accuracy. For thatreason it is usually a mechanically complex and expensive device.Furthermore, the blades of the adjustable field stop have to be imagedon the mask with the help of a large objective. This objective involvesa lot of space and significantly adds to the total manufacturing costsof the illumination system. WO 2010/006687 A1 discloses an illuminationsystem that is similar to the one disclosed in WO 2012/100791 A1 thathas been mentioned above. Here the variable light patterns on the lightentrance facets are not used to modify a field dependence of the angularlight distribution, but to take over the function of the movable bladesof the adjustable field stop. However, also in this prior artillumination system very small light spots have to be produced on thelight entrance facets. These spots are produced by a mirror array atpositions that can be varied by changing a deflection angle produced bythe mirrors.

SUMMARY

It is therefore an object of the present disclosure to provide anillumination system of a microlithographic projection exposure apparatuswhich overcomes at least some of the disadvantages associated with priorart adjustable field stops.

In accordance with the present disclosure this object is achieved by anillumination system that is configured to illuminate a mask moving alonga scan direction in a mask plane. The illumination system includes anoptical integrator configured to produce a plurality of secondary lightsources located in a pupil plane of the illumination system. The opticalintegrator includes a plurality of light entrance facets each beingassociated with one of the secondary light sources. Images of the lightentrance facets at least substantially superimpose in the mask plane.The illumination system further includes a spatial light modulator thathas a light exit surface and is configured to transmit or to reflectimpinging projection light in a spatially resolved manner. A pupilforming unit is configured to direct projection light on the spatiallight modulator. An objective images the light exit surface of thespatial light modulator onto the light entrance facets of the opticalintegrator so that an image of an object area on the light exit surfacecompletely coincides with one of the light entrance facets. Inaccordance with the present disclosure a control unit is configured tocontrol the spatial light modulator such that along the scan direction alength of an image, which is formed on the mask from a light pattern inthe object area, gradually increases at a beginning of a scan cycle andgradually decreases at the end of the scan cycle.

The disclosure is based on the perception that light patterns on thelight entrance facets of the optical integrator are imaged on the mask.By modifying these light patterns it is thus possible to vary the sizeof the illumination field on the mask. For modifying the light patternsa spatial light modulator is used that may be configured as a digitalmirror device (DMD) or a plurality of such devices. Thus the function ofprior art adjustable field stops is taken over by a suitable control ofthe spatial light modulator. The macroscopic movements of the blades inprior art adjustable field stops is replaced, for example, by minutetilt movements of a huge number of digital micromirrors.

In this manner that mechanically complex and expensive adjustable fieldstop can be dispensed with. In accordance with the disclosure there isno need for a field plane (i.e. a plane that is optically conjugate tothe mask) between the light entrance facets and the mask. Thus also thebulky and expensive objective that images the movable blades on the maskis not required.

In comparison to this bulky objective, the additional objective thatimages the light exit surface of the spatial light modulator on thelight entrance facets of the optical integrator has a much smaller sizeand complexity. This is because the light passing through this objectiveis usually almost parallel, which results in a small numerical aperture.Furthermore, the size of the light exit surface and of the opticalintegrator is usually smaller than the size of the illumination field onthe mask. Thus the two quantities that have the main impact on the sizeand complexity of objectives, namely numerical aperture and field size,are small as compared to objectives that image the blades on the mask.The objective that images the light exit surface of the spatial lightmodulator on the light entrance facets can therefore be realized withvery few and preferably spherical lenses.

If compared to the prior art illumination system disclosed in WO2010/006687 A1, the main benefit of the illumination system inaccordance with the present disclosure is the use of a spatial lightmodulator having a light exit surface that is imaged on the lightentrance facets. For that reason it is not necessary to move tiny spotsto arbitrary positions on the light entrance facets, as this is the casein the prior art illumination system. Instead, the light modulator onlyneeds to produce variable light patterns on its light exit surface. Aswill be explained below in further detail, this can be accomplished withrelatively simple digital devices such as digital mirror devices (DMD)or LCD panels.

With the spatial light modulator it is also possible to dispense withmechanical complex devices that are used in prior art illuminationsystems to adjust the spatial irradiance distribution along thecross-scan direction.

Since all components of the illumination system may be purelyreflective, the disclosure can principally also be used in EUVillumination systems.

The pupil forming unit may include a diffractive optical element fordefining an irradiance distribution on the spatial light modulator thatis imaged on the light entrance facets of the optical integrator. Forfine adjustments of this irradiance distribution zoom optics and/or apair of axicon elements may be arranged in the light path between thediffractive optical element and the spatial light modulator.

A more flexible setting of the irradiance distribution on the spatiallight modulator is possible if the pupil forming unit includes a firstbeam deflection array of first reflective or transparent beam deflectionelements. Each beam deflection element is configured to illuminate aspot on the spatial light modulator at a position that is variable bychanging a deflection angle produced by the beam deflection element.

The spatial light modulator may be of the transparent or the reflectivetype and may include an array of elements that can be used to attenuate,completely block or deflect impinging light. For example, the spatiallight modulator may be configured as an LCD panel including a twodimensional array of LCD cells whose optical activity can be controlledindividually by the control unit. In modulators of the transparent typethe object area is usually illuminated from its back side.

In one embodiment the spatial light modulator includes a second beamdeflection array of second reflective or transparent beam deflectionelements. Each second beam deflection element is capable to be in an“on”-state, in which it directs impinging light towards the opticalintegrator, and in an “off”-state, in which it directs impinging lightelsewhere, for example on a light absorbing surface. Such a second beamdeflection array may be configured as a digital mirror device which mayinclude millions of individual micromirrors.

Generally the larger the number of second beam deflection elementsarranged in the object area is, the better is the spatial resolutionthat can be used for changing the irradiance distribution on the maskduring the scan cycle. Preferably at least 10, and even more preferablyat least 50, second beam deflection elements are arranged in the objectarea.

In one embodiment centers of adjacent second beam deflection elementsarranged in the object area are aligned along a straight line. An imageof the straight line forms an angle α to a boundary line of the one ofthe light entrance facets, wherein α is distinct from m·45° with m=0, 1,2, 3, . . . . With such an oblique arrangement of the second beamdeflection array with respect to the light entrance facets the distanceis reduced between cross-scan positions in the illumination field atwhich the attenuation is different.

For example, the boundaries of the second beam deflection elements maybe arranged in a first rectangular grid, and boundaries of the lightentrance facets may be arranged in a second rectangular grid. Then theimage of the first rectangular grid formed on the light entrance facetsforms the angle α to the second rectangular grid.

If the mask moves along a scan direction while it is illuminated by theillumination system, the irradiance and angular irradiance distributionat a point on the mask is obtained by integrating the irradiances andangular irradiance distributions during the scan cycle, i.e. while thepoint on the mask moves through the illumination field. If it is desiredto finely adjust the field dependence of the irradiance and angularirradiance distribution, it may be sufficient to provide only a fewsecond beam deflection elements along the scan direction, but a largernumber of second beam deflection elements along the cross-scandirection. This usually implies that a length of the object area alongthe first direction should be larger than a length of the object areaalong a second direction which is orthogonal to the first direction.Then the objective should be an anamorphotic objective having amagnification M with |M| being smaller along the first direction thanalong the second direction. The anamorphotic objective ensures that theimage of the elongated object area is not elongated, but coincides withthe (usually square) shape of the light entrance facets.

Instead of or in addition to using an anamorphotic objective, it ispossible to use an anamorphotic condenser having a front focal planewhich coincides with the pupil plane and having a focal length f beingshorter along the first direction than along the second direction.

Generally it is preferred if the second beam deflection elements arearranged in an object plane of the objective that is parallel to a planein which the light entrance facets are arranged. This can be achieved ifthe second beam deflection elements are configured such that theyproduce in the “on”-state a deflection of impinging light by an angledistinct from zero. Additionally or alternatively the objective may benon-telecentric on an object side and telecentric on an image side.

Generally the light spots produced by the first beam deflection array onthe object area will be larger than the object area. However, thedisclosure may also be used if the spots are smaller than the objectarea.

Since gaps between second beam deflection elements are, via the lightentrance facets of the optical integrator, eventually imaged on theillumination field, measures should be taken that this does notcompromise the uniformity of the spatial and angular irradiancedistribution in the illumination field. To this end a scattering platemay be arranged in a light path between the optical light modulator andthe mask plane, preferably close to a field plane. The scattering plateblurs the irradiance distribution on the light entrance facets and thusensures that no dark lines occur in the illumination field.

If the light exit surface of the optical light modulator includes groupsof object areas that are separated by areas that are not imaged on thelight entrance facets, the objective may be configured to combine imagesof the active object areas so that the images of the object areas abuton the light entrance facets.

In particular the objective may include a first array of first opticalelements, wherein each first optical element forms a magnified image ofone of the object areas in an intermediate image plane, and imagingoptics that image the intermediate image plane on the light entrancefacets.

Usually it is desired that the image, which is formed on the mask fromthe light pattern in the object area, has a length along a cross-scandirection, which is perpendicular to the scan direction, that remainsconstant during the scan cycle. Then the illumination field changes itssize only along the scan direction at the beginning and the end of eachscan cycle.

In one embodiment the control unit is configured to control the spatiallight modulator such that rows of adjacent second beam deflectionelements are simultaneously brought from the “off” state into the “on”state and vice versa during the scan cycle. Then the length of theillumination field along the scan direction mask increases or decreasesuniformly along the cross-scan direction, i.e. the illumination fieldchanges by the addition or deletion of straight or curved narrowstripes.

Subject of the disclosure is also a microlithographic projectionexposure apparatus including the illumination system in accordance withthe present disclosure and a mask. The mask contains a light absorbingstripe extending perpendicular to the scan direction and having a widthalong the scan direction which is greater than or equal to a lengthalong the scan direction of an image of one of the second beamdeflection elements formed on the mask plane. Then a narrow stripe beingan image of one row of second beam deflection elements can beilluminated first on the light absorbing stripe. As the mask movesfurther along the scan direction, the illuminated stripe enters acentral area of the mask where the features to be illuminated arearranged. In this manner the length of the illumination field along thescan direction changes continuously in spite of the discrete characterof the beam deflection elements. The term “light absorbing stripe” is tobe understood broadly in the sense that the stripe may also reflect thelight to a light absorbing element arranged elsewhere.

Subject of the disclosure is also an illumination system of a projectionexposure apparatus including a spatial light modulator that isconfigured to transmit or to reflect impinging light in a spatiallyresolved manner, a pupil forming unit configured to direct light on thespatial light modulator, and an optical integrator configured to producea plurality of secondary light sources located in a pupil plane. Theoptical integrator includes a plurality of light entrance facets eachbeing associated with one of the secondary light sources. An objectiveimages a light exit surface of the spatial light modulator on the lightentrance facets of the optical integrator. In accordance with thepresent disclosure there is no plane that is optically conjugate to themask between the light entrance facets and the mask.

This illumination system thus dispenses with a field stop plane in whichan adjustable field stop including moving blades is arranged. Thefunction of the adjustable field stop is taken over by the spatial lightmodulator.

To this end the illumination system may include a control unit that isconfigured to control the spatial light modulator such that along thescan direction a length of an image, which is formed on the mask from alight pattern in an object area of the spatial light modulator,gradually increases at a beginning of a scan cycle and graduallydecreases at the end of the scan cycle.

Subject of the disclosure is also a method of operating an illuminationsystem of a microlithographic projection exposure apparatus, includingthe following steps:

-   a) illuminating a spatial light modulator having a light exit    surface;-   b) producing a light pattern in an object area on the light exit    surface;-   c) forming an intermediate image of the light pattern on a light    entrance facet of an optical integrator and a final image on a mask;-   d) while the mask moves along a scan direction, changing the light    pattern on the object area so that a length of the final image along    the scan direction gradually increases at the beginning of a scan    cycle and gradually decreases at the end of the scan cycle.

The final image on the mask may have a length along a cross-scandirection, which is perpendicular to the scan direction, that remainsconstant during the scan cycle.

Between the light entrance facets and the mask there may be no planethat is optically conjugate to the mask.

The spatial light modulator may be illuminated by a pupil forming unitthat includes a first beam deflection array of first reflective ortransparent beam deflection elements. Each beam deflection elementilluminates a spot on the spatial light modulator at a position that isvariable by changing a deflection angle produced by the beam deflectionelement.

The spatial light modulator may include a second beam deflection arrayof second reflective or transparent beam deflection elements. Somesecond beam deflection element may then be switched from an “off” stateinto an “on” state at the beginning of the scan cycle, and some secondbeam deflection elements may then be switched from the “on” state to the“off” state at the end of the scan cycle.

Rows of adjacent second beam deflection elements may be simultaneouslybrought from the “off” state into the “on” state at the beginning of thescan cycle, and different rows of adjacent second beam deflectionelements may be switched from the “on” state to the “off” state at theend of the scan cycle.

The light exit surface of the optical light modulator may include groupsof object areas that are separated by areas that are not imaged on thelight entrance facets. An objective combines images of the object areasso that the images of the object areas abut on the optical integrator.

Such an objective may include a first array of first optical elements,wherein each first optical element forms a magnified image of one of thegroups in an intermediate image plane. Imaging optics image theintermediate image plane on the light entrance facets.

DEFINITIONS

The term “light” is used herein to denote any electromagnetic radiation,in particular visible light, UV, DUV, VUV and EUV light and X-rays.

The term “light ray” is used herein to denote light whose path ofpropagation can be described by a line.

The term “light bundle” is used herein to denote a plurality of lightrays that have a common origin in a field plane.

The term “light beam” is used herein to denote all light that passesthrough a particular lens or another optical element.

The term “position” is used herein to denote the location of a referencepoint of a body in the three-dimensional space. The position is usuallyindicated by a set of three Cartesian coordinates. The orientation andthe position therefore fully describe the placement of a body in thethree-dimensional space.

The term “surface” is used herein to denote any plane or curved surfacein the three-dimensional space. The surface may be part of a body or maybe completely separated therefrom, as it is usually the case with afield or a pupil plane.

The term “field plane” is used herein to denote the mask plane or anyother plane that is optically conjugate to the mask plane.

The term “pupil plane” is a plane in which (at least approximately) aFourier relationship is established to a field plane. Generally marginalrays passing through different points in the mask plane intersect in apupil plane, and chief rays intersect the optical axis. As usual in theart, the term “pupil plane” is also used if it is in fact not a plane inthe mathematical sense, but is slightly curved so that, in the strictsense, it should be referred to as pupil surface.

The term “uniform” is used herein to denote a property that does notdepend on the position.

The term “optical raster element” is used herein to denote any opticalelement, for example a lens, a prism or a diffractive optical element,which is arranged, together with other identical or similar opticalraster elements so that each optical raster element is associated withone of a plurality of adjacent optical channels.

The term “optical integrator” is used herein to denote an optical systemthat increases the product NA·a, wherein NA is the numerical apertureand a is the illumination field area.

The term “condenser” is used herein to denote an optical element or anoptical system that establishes (at least approximately) a Fourierrelationship between two planes, for example a field plane and a pupilplane.

The term “conjugate plane” is used herein to denote planes between whichan imaging relationship is established. More information relating to theconcept of conjugate planes are described in an essay E. Delanoentitled: “First-order Design and the y, y Diagram”, Applied Optics,1963, vol. 2, no. 12, pages 1251-1256.

The term “field dependence” is used herein to denote any functionaldependence of a physical quantity from the position in a field plane.

The term “spatial irradiance distribution” is used herein to denote howthe total irradiance varies over a real or imaginary surface on whichlight impinges. Usually the spatial irradiance distribution can bedescribed by a function I_(s)(x, y), with x, y being spatial coordinatesof a point on the surface.

The term “angular irradiance distribution” is used herein to denote howthe irradiance of a light bundle varies depending on the angles of thelight rays that constitute the light bundle. Usually the angularirradiance distribution can be described by a function I_(a)(α,β), withα, β being angular coordinates describing the directions of the lightrays. If the angular irradiance distribution has a field dependence,I_(a) will be also a function of field coordinates, i.e. I_(a)=I_(a)(α,β,x,y). The field dependence of the angular irradiance distribution maybe described by a set of expansion coefficients a_(ij) of a Taylor (oranother suitable) expansion of I_(a)(α, β,x,y) in x, y.

The term “scan cycle” is used herein to denote a scanning process duringwhich a complete die on the wafer or another support is exposed toprojection light.

BRIEF DESCRIPTION OF THE DRAWINGS

Various features and advantages of the present disclosure may be morereadily understood with reference to the following detailed descriptiontaken in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic perspective view of a projection exposureapparatus in accordance with one embodiment of the present disclosure;

FIG. 2 is an enlarged perspective view of the mask to be projected bythe projection exposure apparatus shown in FIG. 1, illustrating variousdeficiencies of the angular irradiance distribution;

FIG. 3 is a meridional section through an illumination system being partof the apparatus shown in FIG. 1;

FIG. 4 is a perspective view of a first mirror array contained in theillumination system shown in FIG. 3;

FIG. 5 is a perspective view of a second mirror array contained in theillumination system shown in FIG. 3;

FIG. 6 is a perspective view of an optical integrator contained in theillumination system shown in FIG. 3;

FIG. 7 is a schematic meridional section through the first and thesecond mirror array shown in FIGS. 4 and 5;

FIG. 8 is a perspective view on the second mirror array shown in FIG. 5,but illuminated with two poles;

FIG. 9 is a perspective view of the optical integrator shown in FIG. 6,but illuminated with two poles;

FIG. 10 is a schematic meridional section through a portion of theillumination system in which only a mirror array, a condenser and anarray of optical raster elements are shown;

FIGS. 11 a and 11 b are top views on the second mirror array and theoptical integrator shown in FIG. 3;

FIG. 12 illustrates an irradiance distribution on a light entrance facetof the optical integrator;

FIG. 13 is a graph showing the scan integrated irradiance distributionalong the X direction produced by the light entrance facet shown in FIG.12;

FIG. 14 illustrates another irradiance distribution on a light entrancefacet of the optical integrator;

FIG. 15 is a graph showing the scan integrated irradiance distributionalong the X direction produced by the light entrance facet shown in FIG.14;

FIG. 16 is a top view on the second mirror array on which a plurality oflight spots produce an irradiance distribution;

FIG. 17 shows the second mirror array of FIG. 16, but with several ofthe micromirrors in an “off”-state;

FIG. 18 is a top view on the irradiance distribution on a single lightentrance facet for an alternative embodiment;

FIG. 19 is a graph showing the scan integrated irradiance distributionalong the X direction produced by the light entrance facet shown in FIG.18;

FIGS. 20 a to 20 c illustrate images of micromirrors on a light entrancefacet and the corresponding irradiance distribution on the mask;

FIG. 21 is a graph showing the total irradiance distribution that isobtained by superimposing the irradiance distributions shown in FIGS. 20a to 20 c;

FIG. 22 is a schematic meridional section through an objective, which iscontained in the illumination system shown in FIG. 3, and an additionalscattering plate;

FIG. 23 is a schematic perspective view on an object area on the secondmirror array, an anamorphotic objective and an optical raster element ofthe optical integrator;

FIG. 24 is a schematic meridional section showing the second mirrorarray, the objective and a light entrance facet;

FIG. 25 shows a similar arrangement as in FIG. 24, but with an off-axisarrangement of the micromirrors and the light entrance facets;

FIG. 26 is a meridional section through an embodiment in which groups ofobject areas are separated by a gap that is not imaged on the lightentrance facets;

FIG. 27 is a meridional section through an illumination system accordingto another embodiment in which a diffractive optical element is used todefine the irradiance distribution on an LCD panel used as spatial lightmodulator;

FIGS. 28 a to 28 h are top views on an object area of the spatial lightmodulator, on a light entrance facet and on the mask at various stagesof a scan cycle;

FIG. 29 is a flow diagram that illustrates important method steps.

DESCRIPTION OF THE PREFERRED EMBODIMENTS I General Construction ofProjection Exposure Apparatus

FIG. 1 is a perspective and highly simplified view of a projectionexposure apparatus 10 in accordance with the present disclosure. Theapparatus 10 includes a light source 11 that may be realized as anexcimer laser, for example. The light source 11 in this embodimentproduces projection light having a center wavelength of 193 nm. Otherwavelengths, for example 257 nm or 248 nm, are envisaged as well.

The apparatus 10 further includes an illumination system 12 whichconditions the projection light provided by the light source 11 in amanner that will be explained below in further detail. The projectionlight emerging from the illumination system 12 illuminates anillumination field 14 on a mask 16. The mask 16 contains a pattern 18formed by a plurality of small features 19 that are schematicallyindicated in FIG. 1 as thin lines. The features 19 are surrounded by anopaque rim 150, as this will be explained in further detail below withreference to FIGS. 28 a to 28 h. In this embodiment the illuminationfield 14 has the shape of a rectangle. However, other shapes of theillumination field 14, for example a ring segment, are alsocontemplated.

A projection objective 20 including lenses L1 to L6 images the pattern18 within the illumination field 14 onto a light sensitive layer 22, forexample a photoresist, which is supported by a substrate 24. Thesubstrate 24, which may be formed by a silicon wafer, is arranged on awafer stage (not shown) such that a top surface of the light sensitivelayer 22 is precisely located in an image plane of the projectionobjective 20. The mask 16 is positioned via a mask stage (not shown) inan object plane of the projection objective 20. Since the latter has amagnification β with |β|<1, a minified image 18′ of the pattern 18within the illumination field 14 is projected onto the light sensitivelayer 22.

During the projection the mask 16 and the substrate 24 move along a scandirection which corresponds to the Y direction indicated in FIG. 1. Theillumination field 14 then scans over the mask 16 so that patternedareas larger than the illumination field 14 can be continuously imaged.The ratio between the velocities of the substrate 24 and the mask 16 isequal to the magnification β of the projection objective 20. If theprojection objective 20 does not invert the image (β>0), the mask 16 andthe substrate 24 move along the same direction, as this is indicated inFIG. 1 by arrows A1 and A2. However, the present disclosure may also beused in stepper tools in which the mask 16 and the substrate 24 do notmove during projection of the mask.

II Field Dependent Angular Irradiance Distribution

FIG. 2 is an enlarged perspective view of the mask 16 containing anotherexemplary pattern 18. For the sake of simplicity it is assumed that thepattern 18 includes only features 19 that extend along the Y direction.It is further assumed that the features 19 extending along the Ydirection are best imaged on the light sensitive layer 22 with an Xdipole illumination setting.

In FIG. 2 an exit pupil 26 a associated with a light bundle isillustrated by a circle. The light bundle converges towards a fieldpoint that is located at a certain X position of the illumination field14 at a first time during a scan cycle. In the exit pupil 26 a two poles27 a, which are spaced apart along the X direction, represent directionsfrom which projection light propagates towards this field point. Thelight energies concentrated in each pole 27 a are assumed to be equal.Thus the projection light impinging from the +X direction has the sameenergy as the projection light impinging from the −X direction. Sincethe features 19 are assumed to be uniformly distributed over the pattern18, this X dipole illumination setting should be produced at each fieldpoint on the mask 16.

Another exit pupil denoted by 26 b is associated with a light bundlethat converges towards a field point that is located at another Xposition of the illumination field 14 at a later time of the scan cycle.The light energies concentrated in each pole 27 b are again equal.However, the light associated with the poles 27 b are tilted compared tothe light cones of light that are associated with the ideal pole 27 a.This means that the field point receives the same amount of projectionlight, but the directions from which the projection light impinges onthe field point are not ideal for imaging the features 19 on the lightsensitive layer 22.

A further exit pupil denoted by 26 c is associated with a point in theillumination field 14 that is located at still another X position. Hereit is assumed that the directions from which the projection lightimpinges on the field point are again ideal for imaging the features 19.Therefore also the light cones associated with the poles 27 c have thesame cone angle and orientation as the cones associated with the idealexit pupil 26 a. However, the poles 27 c are not balanced, i.e. thelight energy concentrated in the poles 27 c differs from one another.Thus the projection light impinging from the +X direction has lessenergy than the projection light impinging from the −X direction.

From the foregoing it becomes clear that the ideal angular irradiancedistribution represented by the exit pupil 26 a is not obtained at eachX position in the illumination field 14. The angular irradiancedistribution is therefore field-dependent, i.e. at different fieldpoints the angular irradiance distribution is different.

A field dependence may not only occur along the X direction, but alsoalong the Y direction within the illumination field 14. Then one pointon the mask 16 experiences different angular irradiance distributionswhile it passes through the illumination field 14 during a scan cycle.If a field dependence along the Y direction (i.e. the scan direction)occurs, it has to be taken into account that the total effect for aparticular field point is obtained by integrating the different angularirradiance distributions.

There is a wide variety of further field-dependent deviations of a realangular irradiance distribution from the ideal one. For example, thepoles in the exit pupil associated with some field points may bedeformed, blurred or may not have a desired non-uniform irradiancedistribution.

If field dependent deviations from the ideal angular irradiancedistribution occur, this generally has a negative impact on the qualityof the pattern image that is formed on the light sensitive layer 22. Inparticular, the dimensions of the structures that are produced with thehelp of the apparatus 10 may vary inadvertently, and this may compromisethe function of the devices containing these structures. Therefore it isgenerally desired to eliminate any field dependence of the illuminationsetting in the illumination field 14.

Sometimes, however, it is desirable to deliberately introduce a fielddependence of the angular irradiance distribution. This may beexpedient, for example, if the projection objective 20 or the mask 16have field depending properties that affect the image of the pattern 18on the light sensitive layer 22. Variations of the imaging properties ofthe projection objective 20 may occur as a result of manufacturingtolerances, aging phenomena or non-uniform temperature distributions,for example. A field dependence of the mask 16 often occurs as a resultof features that have different orientations or dimensions, for example.Often field dependent adverse effects can be successfully reduced byselectively introducing a field dependence of the angular irradiancedistribution. Since some of these effects change very rapidly, it issometimes desired to change the field dependence of the angularirradiance distribution during a single scan cycle.

III General Construction of Illumination System

FIG. 3 is a meridional section through the illumination system 12 shownin FIG. 1. For the sake of clarity, the illustration of FIG. 3 isconsiderably simplified and not to scale. This particularly implies thatdifferent optical units are represented by one or very few opticalelements only. In reality, these units may include significantly morelenses and other optical elements.

In the embodiment shown, the projection light emitted by the lightsource 11 enters a beam expansion unit 32 which outputs an expanded andalmost collimated light beam 34. To this end the beam expansion unit 32may include several lenses or may be realized as a mirror arrangement,for example.

The projection light beam 34 then enters a pupil forming unit 36 that isused to produce variable spatial irradiance distributions in asubsequent plane. To this end the pupil forming unit 36 includes a firstmirror array 38 of very small mirrors 40 that can be tilted individuallyabout two orthogonal axes with the help of actuators. FIG. 4 is aperspective view of the first mirror array 38 illustrating how twoparallel light beams 42, 44 are reflected into different directionsdepending on the tilting angles of the mirrors 40 on which the lightbeams 42, 44 impinge. In FIGS. 3 and 4 the first mirror array 38includes only 6×6 mirrors 40; in reality the first mirror array 38 mayinclude several hundreds or even several thousands mirrors 40.

The pupil forming unit 36 further includes a prism 46 having a firstplane surface 48 a and a second plane surface 48 b that are bothinclined with respect to an optical axis OA of the illumination system12. At these inclined surfaces 48 a, 48 b impinging light is reflectedby total internal reflection. The first surface 48 a reflects theimpinging light towards the mirrors 40 of the first mirror array 38, andthe second surface 48 b directs the light reflected from the mirrors 40towards an exit surface 49 of the prism 46. The angular irradiancedistribution of the light emerging from the exit surface 49 can thus bevaried by individually tilting the mirrors 40 of the first mirror array38. More details with regard to the pupil forming unit 36 can be gleanedfrom US 2009/0116093 A1.

The angular irradiance distribution produced by the pupil forming unit36 is transformed into a spatial irradiance distribution with the helpof a first condenser 50. The condenser 50, which may be dispensed within other embodiments, directs the impinging light towards a digitalspatial light modulator 52 that is configured to reflect impinging lightin a spatially resolved manner. To this end the digital spatial lightmodulator 52 includes a second mirror array 54 of micromirrors 56 thatare arranged in a mirror plane 57 and can be seen best in the enlargedcut-out C of FIG. 3 and the enlarged cut-out C′ of FIG. 5. In contrastto the mirrors 40 of the first mirror array 38, however, eachmicromirror 56 of the second mirror array 54 has only two stableoperating states, namely an “on” state, in which it directs impinginglight via a first objective 58 towards an optical integrator 60, and an“off” state, in which it directs impinging towards a light absorbingsurface 62.

The second mirror array 54 may be realized as a digital mirror device(DMD), as they are commonly used in beamers, for example. Such devicesmay include up to several million micromirrors that can be switchedbetween the two operating states many thousands times per second.

Similar to the pupil forming unit 36, the spatial light modulator 52further includes a prism 64 having an entrance surface 65 that isarranged perpendicular to the optical axis OA and a first plane surface66 a and a second plane surface 66 b that are both inclined with respectto the optical axis OA of the illumination system 12. At these inclinedsurfaces 66 a, 66 b impinging light is reflected by total internalreflection. The first surface 66 a reflects the impinging light towardsthe micromirrors 56 of the second mirror array 54, and the secondsurface 66 b directs the light reflected from the micromirrors 56towards a surface 68 of the prism 64.

If all micromirrors 56 of the second mirror array 54 are in their “on”state, the second mirror array 54 has substantially the effect of aplane beam folding mirror. However, if one or more micromirrors 56 areswitched to their “off” state, the spatial irradiance distribution ofthe light emerging from the mirror plane 57 is modified. This can beused, in a manner that will be explained further below in more detail,to produce a field dependent modification of the angular lightdistribution on the mask 16.

As it already has been mentioned above, the light emerging from theprism 64 passes through the first objective 58 and impinges on theoptical integrator 60. Since the light passing through the firstobjective 58 is almost collimated, the first objective 58 may have avery low numerical aperture (for example 0.01 or even below) and thuscan be realized with a few small spherical lenses. The first objective58 images the mirror plane 57 of the spatial light modulator 52 onto theoptical integrator 60.

The optical integrator 60 includes, in the embodiment shown, a firstarray 70 and a second array 72 of optical raster elements 74. FIG. 6 isa perspective view of the two arrays 70, 72. Each array 70, 72 includes,on each side of a support plate, a parallel array of cylinder lensesextending along the X and the Y direction, respectively. The volumeswhere two cylinder lenses cross form optical raster elements 74. Thuseach optical raster element 74 may be regarded as a microlens havingcylindrically curved surfaces. The use of cylinder lenses isadvantageous particularly in those cases in which the refractive powerof the optical raster elements 74 shall be different along the X and theY direction. A different refractive power is used if the squareirradiance distribution on the optical integrator 60 shall betransformed into a slit-shaped illumination field 14, as this is usuallythe case. The surface of the optical raster elements 74 pointing towardsthe spatial light modulator 52 will be referred to in the following aslight entrance facet 75.

The optical raster elements 74 of the first and second array 70, 72respectively, are arranged one behind the other in such a way that oneoptical raster element 74 of the first array 70 is associated in a oneto one correspondence with one optical raster element 74 of the secondarray 72. The two optical raster elements 74, which are associated witheach other, are aligned along a common axis and define an opticalchannel. Within the optical integrator 60 a light beam which propagatesin one optical channel does not cross or superimpose with light beamspropagating in other optical channels. Thus the optical channelsassociated with the optical raster elements 74 are optically isolatedfrom each other.

In this embodiment a pupil plane 76 of the illumination system 12 islocated behind the second array 72; however, it may equally be arrangedin front of it. A second condenser 78 establishes a Fourier relationshipbetween the pupil plane 76 and a mask plane 88 in which the mask 16moves along the scan direction Y during a scan process scan cycle.Between the light entrance facets 75 of the optical integrator 60 andthe mask 16 there is no field plane, i.e. no plane that is opticallyconjugate to the mask 16. This is a remarkable feature of theillumination system 12, because conventional illumination systemsusually have an adjustable field stop that includes movable blades andis arranged in a field plane between the optical integrator 60 and themask 16. The movable blades ensure that the illumination field 14,synchronized with the mask movement, opens and closes along the scandirection Y at the beginning and the end of each scan cycle,respectively. As it will become apparent from section IV.11 below, thisfunction is performed in the illumination system 12 by a suitablecontrol of the spatial light modulator 52.

The mask plane 88 is thus optically conjugate to a raster field plane 84which is located within or in close proximity to the light entrancefacets 75 of the optical integrator 60. This means that an irradiancedistribution on each light entrance facet 75 in the raster field plane84 is imaged onto the mask plane 88 by the associated optical rasterelement 74 of the second array 72 and the second condenser 78. Theimages of the irradiance distributions (or light pattern) on the lightentrance facet 75 within all optical channels superimpose in the maskplane 88, which results in a very uniform illumination of the mask 16.

Another way of describing the uniform illumination of the mask 16 isbased on the irradiance distribution which is produced by each opticalchannel in the pupil plane 76. This irradiance distribution is oftenreferred to as secondary light source. All secondary light sourcescommonly illuminate the mask plane 88 with projection light fromdifferent directions. If a secondary light source is “dark”, no lightimpinges on the mask 16 from a (small) range of directions that isassociated with this particular light source. Thus it is possible to setthe desired angular light distribution on the mask 16 by simplyswitching on and off the secondary light sources formed in the pupilplane 76. This is accomplished by changing the irradiance distributionon the optical integrator 60 with the help of the pupil forming unit 36.

The pupil forming unit 36 and the spatial light modulator 52 areconnected to a control unit 90 which is, in turn, connected to anoverall system control 92 illustrated as a personal computer. Thecontrol unit 90 is configured to control the mirrors 40 of the pupilforming unit 36 and the micromirrors 56 of the spatial light modulator52 in such a manner that the angular irradiance distribution in the maskplane 88 is uniform, or a desired field dependence angular irradiancedistribution is obtained, and that the illumination field 14 opens andcloses along the scan direction Y at the beginning and the end of thescan cycle.

In the following it will be described how this is accomplished.

IV Function and Control of the Illumination System 1. Pupil Forming

FIG. 7 schematically illustrates how the pupil forming unit 36 producesan irradiance distribution on the micromirrors 56 of the spatial lightmodulator 52. For the sake of simplicity the prisms 46, 64 are notshown.

Each mirror 40 of the first mirror array 38 is configured to illuminatea spot 94 on the mirror plane 57 of the spatial light modulator 52 at aposition that is variable by changing a deflection angle produced by therespective mirror 40. Thus the spots 94 can be freely moved over themirror plane 57 by tilting the mirrors 40 around their tilt axes. Inthis way it is possible to produce a wide variety of differentirradiance distributions on the mirror plane 57. The spots 94 may alsopartly or completely overlap, as this is shown at 95. Then also gradedirradiance distributions may be produced.

FIG. 8 is a perspective view, similar to FIG. 5, on the second mirrorarray 54 contained in the spatial light modulator 52. Here it is assumedthat the pupil forming unit 36 has produced an irradiance distributionon the second mirror array 54 that consists of two square poles 27 eachextending exactly over 6×6 micromirrors 56. The poles 27 are arrangedpoint-symmetrically along the X direction.

The objective 58 forms an image of this irradiance distribution on thelight entrance facets 75 of the optical integrator 60, as this is shownin FIG. 9. Here it is assumed that all micromirrors 56 are in the“on”-state so that the irradiance distribution formed on the secondmirror array 54 is identically reproduced (apart from a possible scalingdue to a magnification of the objective 58) on the light entrance facets75 of the optical integrator 60. For the sake of simplicity images ofgaps that separate adjacent micromirrors 56 of the second mirror array54 are disregarded. The regular grid shown on the light entrance facets75 represent an image of the borderlines of the micromirrors 56, butthis image does not appear outside the poles 27 and is shown only inFIG. 9 for illustrative reasons.

2. Field Dependence

Since the light entrance facets 75 are located in the raster field plane84, the irradiance distribution on the light entrance facets 75 isimaged, via the optical raster elements 74 of the second array 72 andthe second condenser 78, on the mask plane 88.

This will now be explained with reference to FIG. 10 which is anenlarged and not to scale cut-out from FIG. 3. Here only two pairs ofoptical raster elements 74 of the optical integrator 60, the secondcondenser 78 and the mask plane 88 are shown schematically.

Two optical raster elements 74 that are associated with a single opticalchannel are referred to in the following as first microlens 101 andsecond microlens 102, respectively. The microlenses 101, 102 aresometimes referred to as field and pupil honeycomb lenses. Each pair ofmicrolenses 101, 102 associated with a particular optical channelproduces a secondary light source 106 in the pupil plane 76. In theupper half of FIG. 10 it is assumed that converging light bundles L1 a,L2 a and L3 a illustrated with solid, dotted and broken lines,respectively, impinge on different points of the light entrance facet 75of the first microlens 101. After having passed the two microlenses 101,102 and the condenser 78, each light bundle L1 a, L2 a and L3 aconverges to a focal point F1, F2 and F3, respectively. From the upperhalf of FIG. 10 it becomes clear that points, where light rays impingeon the light entrance facet 75, and points where these light rays passthe mask plane 88 are optically conjugate.

The lower half of FIG. 10 illustrates the case when collimated lightbundles L1 b, L2 b and L3 b impinge on different regions of the lightentrance facet 75 of the first microlens 101. This is the more realisticcase because the light impinging on the optical integrator 60 is usuallysubstantially collimated. The light bundles L1 b, L2 b and L3 b arefocused in a common focal point F located in the second microlens 102and then pass, now collimated again, the mask plane 88. Again it can beseen that, as a result of the optical conjugation, the region where alight bundle L1 b, L2 b and L3 b impinges on the light entrance facet 75corresponds to a portion of the illumination field 14 in the mask plane88. As a matter of course, these considerations apply separately for theX and the Y direction if the microlenses 101, 102 have refractive powerboth along the X and Y direction.

Therefore each point on a light entrance facet 75 directly correspondsto a conjugate point in the illumination field 14 in the mask plane 88.If it is possible to selectively influence the irradiance on a point ona light entrance facet 75, it is thus possible to influence theirradiance of a light ray that impinges on the conjugate point in themask plane 88 from a direction that depends on the position of the lightentrance facet 75 with respect to the optical axis OA of theillumination system. The larger the distance between a particular lightentrance facet 75 from the optical axis OA is, the larger is the angleunder which the light ray impinges on the point on the mask 16.

3. Modifying Irradiance on Light Entrance Facets

In the illumination system 12 the spatial light modulator 52 is used tomodify the irradiance on points on the light entrance facets 75. In FIG.9 it can be seen that each pole 27 extends over a plurality of smallareas that are images of the micromirrors 56. If a micromirror isbrought into an “off” state, the conjugate area on the light entrancefacet 75 will not be illuminated, and consequently no projection lightwill impinge on a conjugate area on the mask from the (small) range ofdirections that is associated with this particular light entrance facet75.

This will be explained in more detail with reference to FIGS. 11 a and11 b which are top views on the micromirrors 56 of the spatial lightmodulator 52 and on the light entrance facets 75 of the opticalintegrator 60, respectively.

The thick dotted lines on the second mirror array 54 divide its mirrorplane 57 into a plurality of object areas 110 each including 3×3micromirrors 56. The objective 58 forms an image of each object area 110on the optical integrator 60. This image will be referred to in thefollowing as image area 110′. Each image area 110′ completely coincideswith a light entrance facet 75, i.e. the image areas 110′ have the sameshape, size and orientation as the light entrance facets 75 and arecompletely superimposed on the latter. Since each object area 110includes 3×3 micromirrors 56, the image areas 110′ also include 3×3images 56′ of micromirrors 56.

In FIG. 11 a there are eight object areas 110 that are completelyilluminated by the pupil forming unit 36 with projection light. Theseeight object areas 110 form the two poles 27. It can be seen that insome of the object areas 110 one, two or more micromirrors 56 drepresented as black squares have been controlled by the control unit 90such that they are in an “off”-state in which impinging projection lightis not directed towards the objective 58, but towards the absorber 62.By switching micromirrors between the “on” and the “off” state it isthus possible to variably prevent projection light from impinging oncorresponding regions within the image areas 110′ on the light entrancefacets 75, as this is shown in FIG. 11 b. These regions will be referredto in the following as dark spots 56 d′.

As has been explained above with reference to FIG. 10, the irradiancedistribution on the light entrance facets 75 is imaged on the mask plane88. If a light entrance facet 75 contains one or more dark spots 56 d′,as this is illustrated in the upper portion of FIG. 12, the irradiancedistribution produced in the mask plane 88 by the associated opticalchannel will have dark spots at certain X positions, too. If a point ona mask passes through the illumination field 14, the total scanintegrated irradiance will thus depend on the X position of the point inthe illumination field 14, as this is shown in the graph of FIG. 13.Points in the middle of the illumination field 14 will experience thehighest scan integrated irradiance, because they do not pass throughdark spots, and points at the longitudinal ends of the illuminationfield 14 will receive total irradiances that are reduced to differentextents. Thus the field dependence of the angular light distribution onthe mask 16 can be modified by selectively bringing one or moremicromirrors 56 of the spatial light modulator 52 from an “on”-stateinto the “off”-state.

In a foregoing it has to be assumed that each object area 110, which isimaged on one of the light entrance facets 75, contains only 3×3micromirrors 56. Thus the resolution along the cross-scan direction Xthat can be used to modify the field dependence of the angular lightdistribution is relatively coarse. If the number of micromirrors 56within each object area 110 is increased, this resolution can beimproved.

FIG. 14 illustrates a top view on one of the light entrance facets 75for an embodiment in which 20×20 micromirrors 56 are contained in eachobject area 110. Then more complicated scan integrated irradiancedistributions along the X direction can be achieved on the mask 16, asthis is illustrated in the graph shown in FIG. 15.

4. Clipping

In the foregoing it has been assumed that the pupil forming unit 36illuminates poles 27 on the second mirror array 54 that exactly extendover four adjacent object areas 110. Generally, however, it will bedifficult to produce such an irradiance distribution with sharp edges.

The spatial light modulator 52 may also be used to clip a blurredirradiance distribution in the mirror plane 57 by bringing thosemicromirrors 56 into the “off”-state that lie outside the object areas110 that shall be illuminated.

This is illustrated in FIGS. 16 and 17 in which an irradiancedistribution 96 on the second mirror array 54 are shown. Here it isassumed that the movable light spots 94 produced by the mirrors 40 ofthe pupil forming unit 36 are superimposed to form four poles. If allmicromirrors 56 of the spatial light modulator 52 are in the “on” stateas shown in FIG. 16, the blurred irradiance distribution 96 would beimaged on the light entrance facets 75. If those micromirrors 56surrounding the desired object areas 110 are brought into the“off”-state as shown in FIG. 17, they form a frame that delimits thepoles and thus produces sharp edges of the intensity distribution on thelight entrance facets.

5. Relative Rotation

In the embodiments described so far it has been assumed that themicromirrors 56 are aligned parallel to the borderlines of the objectareas 110. The rectangular grid formed by the micromirrors 56 is thenparallel to the rectangular grid which is formed by the light entrancefacets 75. This results in irradiance distributions as shown in FIGS. 13and 15 in which the irradiance along one “column” of micromirrors 56 isalways uniform. Thus only stepped irradiance distributions can beproduced on the light entrance facets 75.

Sometimes it is desirable to produce irradiance distributions that arenot stepped, but contain inclined portions. This can be achieved if thetwo rectangular grids are not arranged parallel to each other, but withan angle α, as this is shown in FIG. 18. Here the images 56′ of themicromirrors 56 form a grid 114 which forms an angle α with the lateralsides of the light entrance facet 75. Then the centers of adjacentmicromirrors 56 are aligned along a straight line having an image 116that forms the same angle α to a boundary line of the light entrancefacet 75. If this angle α is distinct from m·45° with m=0, 1, 2, 3, . .. , the irradiance distribution will not have the shape of steppedprofiles as shown in FIGS. 13 and 15.

FIG. 19 is a graph that illustrates the scan integrated irradiancedistribution along the X direction for the rotated arrangement shown inFIG. 18. Some particular X positions are indicated in FIG. 18 withbroken lines. If the angle α is distinct from m·45° with m=0, 1, 2, 3, .. . , the degeneration is reduced so that a desired attenuation can beobtained at more different X positions. In other words, it is thuspossible to effectively increase the resolution along the X directionthat is available to modify the field dependence of the angularirradiance distribution.

6a. Gaps—Lateral Displacement

As mentioned further above, it is usually inevitable that small gaps areformed between adjacent micromirrors 56 of the second mirror array 38.Images of these gaps are formed on the light entrance facets 75 and alsoon the mask 16. If these images extend parallel to the cross-scandirection X, this is of little concern because of the integrating effectthat results from the scan operation. However, dark lines extendingparallel to the scan direction Y could not be compensated by theintegrating effect.

FIG. 20 a shows in the upper portion a top view on one of the lightentrance facets 75 in which the images of the gaps are denoted by 118′.The graph in the lower portion of FIG. 20 a illustrates the irradiancedistribution along the cross-scan direction X that is produced by thisparticular light entrance facet 75 in the mask plane 88. If all lightentrance facets 75 would produce dark lines 120 at the same X positions,no projection light would reach points on the mask 16 at thesepositions.

FIGS. 20 b and 20 c show other light entrance facets 75 in which the gapimages 118′ are laterally displaced along the cross-scan direction X todifferent degrees. Consequently also the dark lines 120 in theirradiance distributions shown in the lower portion of these figures arelaterally displaced. Since the irradiance distributions produced by eachoptical channel are superimposed in the mask plane 88, the dark lines120 are averaged out, as this is shown in FIG. 21. The larger the numberof light entrance facets 75 is and the smaller the dark lines 120 are,the more approximates the irradiance distribution I(x) in the mask plane88 a uniform distribution.

6b. Gaps—Scattering Plate

Alternatively or additionally, a scattering plate 122 may be arranged inan optical path between the optical light modulator 52 and the maskplane 88 in order to avoid dark lines on the mask plane 88 caused by gapimages 118′. Suitable positions of the scattering plate 122 are betweenthe optical light modulator 52 and the objective 58, between theobjective 58 and the optical integrator 60, or in the vicinity of thefield stop plane 80.

FIG. 22 is a schematic meridional section showing several micromirrors56 of the spatial light modulator 52, the objective 58 and thescattering plate 122 arranged in between. A gap 118 between two adjacentmicromirrors 56 is assumed to have a width d, and the axial distancebetween the scattering plate 122 and the light exit surface 57 of thespatial light modulator 52 is denoted by b. If the characteristicscattering angle β of the scattering plate 122 is approximately d/b, theimage of the gap 118 formed on the light entrance facet 75 issufficiently blurred. If the scattering angle β is significantly largerthan d/b, the desired spatial resolution for the field dependence of theirradiance and the angular irradiance distribution is reduced. If thescattering angle β is too small, the images of the gaps will still beprominent on the light entrance facets 75.

7. Rectangular Object Areas

In the embodiments described above it has been assumed that the numberof micromirrors 56 along the scan direction Y and the cross-scandirection X is identical. Then a rectangular grid of square micromirrors56 perfectly fits into a square light entrance facet 75 of the opticalintegrator 60.

The number N_(X) of micromirrors 56 along the cross-scan direction Xdetermines the resolution that is available for adjusting the fielddependence of the irradiance and the angular irradiance distribution.This number should be as high as possible.

The number N_(Y) of micromirrors 56 along the scan direction Y may besignificantly smaller because of the integrating effect caused by thescan operation. Illustratively speaking, a plurality of optical channelsadjacent along the scan direction Y may contribute to the reduction ofthe irradiance on a point on the mask 16 during a scan cycle. This doesnot apply to optical channels that are adjacent along the cross-scandirection X.

These requirements suggest that the object area 110 may well berectangular, with the length along the cross-scan direction X beinglarger (for example two times and preferably at least five times larger)than the length of the object area along the scan direction Y. Assumingmicromirrors 56 having equal dimensions along the directions X and Y,this implies that the number N_(X) of micromirrors 56 along thecross-scan direction X is larger than the number N_(Y) along the scandirection Y.

If a rectangular object area 110 shall be imaged on a square lightentrance facet 75, the objective 58 has to be anamorphotic. Morespecifically, the absolute value of the magnification M has to besmaller along the cross-scan direction X than along the scan directionY, i.e. |M_(X)|<|M_(Y)|. This is illustrated in FIG. 23 in which twocylinder lenses 124, 126 of the objective 58 are arranged between asingle rectangular object area 110 and the light entrance facet 75 of anoptical raster element 74. If the length of the object area 110 alongthe cross-scan direction X is L_(X) and the length along the scandirection Y is L_(Y), |M_(X)/M_(Y)| should be equal to L_(Y)/L_(X).

A similar result is achieved if not the objective 58, but the subsequentcondenser 78 is anamorphotic so that its focal length f is different forthe X and Y directions. If the objective 58 is rotational symmetric sothat M_(X)=M_(Y), the irradiance distributions on the light entrancefacets 75 will be rectangular with the same aspect ratio L_(X)/L_(Y) asthe object area 110. This rectangular irradiance distribution is thenexpanded by the anamorphotic condenser 78 so that a square irradiancedistribution is obtained in the field stop plane 80 and the subsequentmask plane 88. This approach may involve a redesign of the opticalintegrator 60 because the condenser's different focal lengths along thedirections X, Y have to be compensated by the refractive power of theoptical raster elements 74.

8. Arrangement of Mirror Plane

It is usually preferred if the chief rays of the projection lightimpinge perpendicularly on the optical integrator 60. Then also themirror plane 57, which is imaged by the objective 58 on the lightentrance facets 75, is arranged perpendicularly to the optical axis OA,as this is shown in FIG. 24. In such a parallel arrangement of themicromirrors 56 and the light entrance facets 75 the micromirrors 56have to produce a deflection angle which is distinct from zero if theyare in the “on”-state. This is different to conventional digital mirrordevices (DMD) in which all mirror surfaces are arranged in a singleplane if they are in the “on”-state.

Additionally or alternatively, the second mirror array 54 and the lightentrance facets 75 may be arranged in off-axis regions of the objectfield and the image field of the objective 58, respectively. As it isshown in FIG. 25, it is then possible to use an objective 58 such thatit is not telecentric on the object side, but telecentric on the imageside. This means that chief rays forming an angle with the optical axisOA on the object side are nevertheless parallel to the optical axis OAon the image side.

9. Grouping Object Areas

If the number of micromirrors 56 in each object area 110 and also thenumber of optical channels (and thus of the light entrance facets 75)shall be large, the total number of micromirrors 56 in the second mirrorarray 54 may become huge. Since it might be difficult to provide asingle second mirror array 54 that includes such a huge number ofmicromirrors 56, it is envisaged to split up the second mirror deviceinto several sub-units. More specifically, the second mirror array 54may be combined from several groups of object areas, wherein the groupsare separated from each other by dark areas (i.e. an area from which noprojection light emerges) that are not imaged on the light entrancefacets. Each group may be realized as a single device, for example adigital mirror device (DMD).

FIG. 26 is a schematic meridional section through the second mirrorarray 54 and the objective 58 according to this embodiment. It isassumed that the second mirror array 54 includes two groups 54-1, 54-2each realized as digital mirror device (DMD). Each group 54-1, 54-2includes three object areas 110 that extend over a plurality ofmicromirrors 56. The two groups 54-1, 54-2 are separated by a dark area130 which is absorptive and on which no projection light should bedirected by the pupil forming unit 36.

The objective 58 is configured to combine the images 110′ of the objectareas 110 so that they abut at least substantially seamlessly on theoptical integrator 60. There each image area 110′ completely coincideswith one of the light entrance facets 75. To this end the objective 58produces magnified images of the object areas 110 in an intermediateimage plane 132 with the help of a first array of lenses 134. Theobjective 58 further includes an array of second lenses 136 that isarranged in the intermediate image plane 132. Common imaging optics 138then image the intermediate image plane 134, in which the magnifiedimages of the groups already abut, on the light entrance facets 75 ofthe optical integrator 60. In this way the dark areas 130 between thegroups 54-1, 54-2 is not imaged by the objective 58 on the opticalintegrator 60.

10. Diffractive Optical Element and LCD

FIG. 27 is a meridional section similar to FIG. 3 of an alternativeembodiment of an illumination system 12. In this illumination system thepupil forming unit 52 is replaced by a diffractive optical element 142,zoom optics 144 and a pair of axicon elements 146, 148.

The spatial light modulator 52 in this embodiment is formed by an LCDpanel including a two dimensional array of minute LCD cells whoseoptical activity can be controlled individually by the control unit 90.If the projection light produced by the light source 11 is notsufficiently polarized, an additional polarizer may be inserted in thelight path in front of the spatial light modulator 52.

As a matter of course, the embodiments shown in FIGS. 3 and 27 can alsobe combined in different ways so that, for example, a diffractiveoptical element 142 is used together with the second mirror array 54 asspatial light modulator 52.

11. Field Stop Blade Function

In the illumination system 12 shown in FIG. 3 there is, as it hasalready been mentioned further above, no field plane between the lightentrance facets 75 and the mask plane 88 (see also FIG. 10). Inconventional illumination systems the opening and closing of theillumination field 14 at the beginning and the end of each scan cycle,respectively, is accomplished by movable blades of a field stop arrangedin a field plane between the optical integrator 60 and the mask 16. Inthe illumination system 12 of the present disclosure this function istaken over by a suitable control of the second mirror array 54.

This will be explained in the following with reference to FIGS. 28 a to28 h. These figures show, at different stages during a scan cycle, atthe left hand side a schematic top view on one of the object areas 110on the light exit surface 57 of the second mirror array 54. Here it isassumed that 9×9 digital micromirrors 56 are arranged within the objectarea 110.

Next to the object area 110 a corresponding image area 110′ isschematically shown. Here it is assumed that the objective 58 images theobject area 110 at a reduced scale on one of the light entrance facets75 of the optical integrator 60. Since each light entrance facet 75 isoptically conjugate to the mask plane 88, a light pattern in the imagearea 110′ is reproduced in the mask plane 88. In the embodiment shownthis image is compressed along the scan direction Y because the opticalraster elements 74, 75 have different refractive power along the X and Ydirection. For that reason the fully opened illumination field 14 isslit-shaped although the image fields 110′ on the light entrance facets75 are square.

In order to avoid gaps between adjacent micromirrors 56 causing darkstripes in the illumination field 14, different object areas 110 may belaterally displaced with respect to each other, as this has beenexplained above in section IV.6a, and/or a scattering plate may beprovided, as this has been explained above in section IV.6b. For thesake of simplicity the effects of such gaps are neglected in thefollowing.

At the right end side of each FIGS. 28 a to 28 h the mask 16 is shown.The mask 16 is provided with an opaque rim 150 that surrounds a centralarea 152 containing the small features 19 to be imaged on the lightsensitive surface 22. The opaque rim 150 includes a front side 156 and arear side 158 both extending along the cross-scan direction X, and twolateral sides 160 extending along the scan direction Y. The width of thefront side 156 and the rear side 158 of the opaque rim 150 is denoted byw.

FIG. 28 a illustrates the situation before a scan cycle begins. Allmicromirrors 56 of the second mirror array 54 are in the “off” state,and thus the corresponding image area 110′ on the light entrance facet75 is not illuminated.

Then a first row 162-1 of micromirrors 56 is brought into the “on”state, as this is shown in FIG. 28 b. The light pattern formed on thelight entrance facet 75 is a narrow stripe being an image of the firstrow 162-1. At the very moment when the first row 162-1 of micromirrors56 is brought into the “on” state, the image of the narrow stripe, whichnow forms the illumination field 14, is positioned completely on thefront side 156 of the opaque rim 150. Thus there are not yet anyfeatures 19 illuminated with projection light; only the front side 156of the opaque rim 150 is illuminated, and the projection light iscompletely absorbed by the opaque rim 150.

As the mask 16 moves further along the scan direction Y, the thinillumination field 14 evenly enters the central area 152 that issurrounded by the opaque rim 150. Now the imaging of the features 19begins. If seen along the scan direction Y, the length of the lightpattern formed on the light entrance facet 75, and thus also its imageon the mask 16, increases continuously, while the length in thecross-scan direction X remains constant.

Then the next row 162-2 of micromirrors 56 is brought into the “on”state, as this is shown in FIG. 28 c. As can be seen in the enlargedcutout, one half 164 of the illumination field 14 illuminates only thefront side 156 of the opaque rim 150 where the projection light isabsorbed, and the other half 166 of the illumination field 14illuminates the features 19 in the central area 152 of the mask 16.

Then the next row of micromirrors 56 is brought into the “on” state, andso on. This process is continued until all micromirrors 56 are in the“on” state, as this is shown in FIG. 28 d. Now the illumination field 14has its full length along the scan direction Y. The fully openedillumination field 14 is positioned at this moment so that it abuts thefront side 156 of the opaque rim 150.

As the mask 16 moves further along the scan direction Y, theillumination field 14 now having its full length along the scandirection Y continuously scans over the features 19 in the central area152 of the mask 16, as this is shown in FIG. 28 e. In this manner themask 16 is scanned until the illumination field 14 abuts the rear side158 of the opaque rim 150, as this is shown in FIG. 28 f. Now theillumination field 14 begins to enter the rear side 158 of the opaquerim 150 where the projection light is absorbed. When the image of therow 162-1 of micromirrors 56 is arranged completely on the opaque rim150, the micromirrors 56 of the first row 162-1 are brought into the“off” state. The length of the image of the light pattern on the lightentrance facet 75, i.e. the illumination field 14, along the scandirection Y now decreases.

As soon as the image of the second row 162-2 is arranged completely onthe rear side 158 of the opaque rim 150, the micromirrors 56 of thesecond row 162-2 are brought into the “off” state, and so on.

FIG. 28 g illustrates a state in which the first five rows 162-1 to162-5 have been brought into the “off” state. The length of theillumination field 14 along the scan direction Y has meanwhile decreasedby more than one half.

When all micromirrors 56 are again in the “off” state, the illuminationfield 14 is completely closed, and no light reaches the mask 16, as thisis shown in FIG. 28 h.

V Important Method Steps

Important method steps of the present disclosure will now be summarizedwith reference to the flow diagram shown in FIG. 29.

In a first step S1 a spatial light modulator having a light exit surfaceis illuminated.

In a second step S2 a light pattern in an object area on the light exitsurface is produced.

In a third step S3 an intermediate image of the light pattern on a lightentrance facet of an optical integrator and a final image on a mask areformed.

In a fourth step S4, while the mask moves along a scan direction, thelight pattern on the object area is changed so that a length of thefinal image along the scan direction gradually increases at thebeginning of a scan cycle and gradually decreases at the end of the scancycle.

What is claimed is:
 1. An illumination system configured to illuminatean object moving along a scan direction, the illumination systemcomprising: an optical integrator configured so that, during use of theillumination system, the optical integrator produces a plurality ofsecondary light sources in a pupil plane of the illumination system, theoptical integrator comprising a plurality of light entrance facets, eachlight entrance facet associated with a secondary light source, and theoptical integrator configured so that, during use of the illuminationsystem, images of the light entrance facets at least substantiallysuperimposing in the object plane; a spatial light modulator having alight exit surface, the spatial light modulator configured so that,during use of the illumination system, the spatial light modulatortransmits or reflects impinging projection light in a spatially resolvedmanner; a pupil forming unit configured so that, during use of theillumination system, the pupil forming unit directs projection lightonto the spatial light modulator; an objective configured so that,during use of the illumination system, the objective images the lightexit surface of the spatial light modulator onto the light entrancefacets of the optical integrator so that an image of an object area onthe light exit surface completely coincides with one of the lightentrance facets; and a control unit configured so that, during use ofthe illumination system, the control unit controls the spatial lightmodulator so that along the scan direction a length of an image, whichis formed on the object from a light pattern in the object area,gradually increases at a beginning of a scan cycle and graduallydecreases at the end of the scan cycle, wherein the illumination systemis a microlithographic illumination system.
 2. The illumination systemof claim 1, wherein the pupil forming unit comprises a first beamdeflection array of first reflective or transparent beam deflectionelements, and each beam deflection element is configured so that, duringuse of the illumination system, the beam deflection element illuminatesa spot on the spatial light modulator at a position that is variable bychanging a deflection angle produced by the beam deflection element. 3.The illumination system of claim 1, wherein: the spatial light modulatorcomprises a beam deflection array of reflective or transparent beamdeflection elements; each beam deflection element configurable in an onstate and in an off state; for each beam deflection element, the beamdeflection element is configured so that during use of the illuminationsystem: when in its on state, the beam deflection element directsimpinging projection light toward the optical integrator; and when itsoff state, the beam deflection element directs impinging projectionlight elsewhere.
 4. The illumination system of claim 3, wherein the beamdeflection array comprises a digital mirror device.
 5. The illuminationsystem of claim 3, wherein at least 10 beam deflection elements arearranged in the object area.
 6. The illumination system of claim 3,wherein: centers of adjacent beam deflection elements in the object areaare aligned along a straight line; an image of the straight line definesan angle with respect to a boundary line of the one of the lightentrance facets; and the angle is distinct from m·45° with m=0, 1, 2, 3,. . . .
 7. The illumination system of claim 6, wherein: boundaries ofthe beam deflection elements are arranged in a first rectangular grid;boundaries of the light entrance facets are arranged in a secondrectangular grid; and an image of the first rectangular grid formed onthe light entrance facets forms the angle to the second rectangulargrid.
 8. The illumination system of claim 3, wherein a length of theobject area along a first direction is larger than a length of theobject area along a second direction which is orthogonal to the firstdirection, and the objective is an anamorphotic objective having amagnification M with |M| being smaller along the first direction thanalong the second direction.
 9. The illumination system of claim 8,wherein the second direction corresponds to the scan direction.
 10. Theillumination system of claim 3, further comprising a scattering platedispose so that, during use of the illumination system, the scatteringplate is in a light path between the optical light modulator and theobject plane.
 11. The illumination system of claim 3, wherein thecontrol unit is configured so that, during use of the illuminationsystem, the control unit controls the spatial light modulator so thatrows of adjacent beam deflection elements are simultaneously broughtfrom the off state to the on state and vice versa during the scan cycle.12. The illumination system of claim 2, wherein the light spots producedby the beam deflection elements on the object area are larger than theobject area.
 13. The illumination system of claim 1, wherein: the lightexit surface of the optical light modulator comprises groups of objectareas which are separated by areas that are not imaged on the lightentrance facets during use of the illumination system; the objective isconfigured so that, during use of the illumination system, the objectivecombines images of the object areas so that the images of the objectareas abut on the optical integrator.
 14. The illumination system ofclaim 13, wherein the objective comprises: a first array of firstoptical elements, each first optical element configured so that, duringuse of the illumination system, the first optical element forms amagnified image of one of the groups in an intermediate image plane; andimaging optics configured so that, during use of the illuminationoptics, the imaging optics image the intermediate image plane onto thelight entrance facets.
 15. The illumination system of claim 1, whereinthe control unit is configured so that, during use of the illuminationsystem, the control unit controls the spatial light modulator so thatthe image, which is formed on the object from the light pattern in theobject area, has a length along a cross-scan direction, which isperpendicular to the scan direction, that remains constant during thescan cycle.
 16. The illumination system of claim 1, wherein, between thelight entrance facets and the object, there is no plane that isoptically conjugate to the object.
 17. An apparatus, comprising: anillumination system according to claim 1; and a projection objective,wherein the apparatus is a microlithographic projection exposureapparatus.
 18. A method of using an apparatus comprising an illuminationsystem and a projection objective, the method comprising: using theillumination system to illuminate at least some structures of a mask;and using the projection objective to image at least a portion of theilluminated structures onto a light-sensitive material, wherein theillumination system is an illumination system according to claim
 1. 19.An apparatus, comprising: an illumination system according to claim 3;and a projection objective, wherein the apparatus is a microlithographicprojection exposure apparatus.
 20. The apparatus of claim 19, whereinthe object comprises a mask with a light absorbing stripe extendingperpendicular to the scan direction and having a width along the scandirection which is greater than or equal to a length along the scandirection of an image of one of the beam deflection elements formed onthe object plane during use of the apparatus.
 21. An illumination systemconfigured to illuminate an object, the illumination system comprising:a spatial light modulator configured so that, during use of theillumination system, the spatial light modulator transmits or toreflects impinging light in a spatially resolved manner; a pupil formingunit configured so that, during use of the illumination system, thepupil forming unit directs light on the spatial light modulator; anoptical integrator configured so that, during use of the illuminationsystem, the optical integrator produces a plurality of secondary lightsources located in a pupil plane, the optical integrator comprising aplurality of light entrance facets, each light entrance facet beingassociated with one of the secondary light sources; and an objectiveconfigured so that, during use of the illumination system, the objectiveimages a light exit surface of the spatial light modulator onto thelight entrance facets of the optical integrator, wherein: between thelight entrance facets and the object plane, there is no plane that isoptically conjugate to the object plane; and the illumination system isa microlithographic illumination system.
 22. A method, comprising:illuminating a spatial light modulator having a light exit surface;producing a light pattern in an object area on the light exit surface;forming an intermediate image of the light pattern on a light entrancefacet of an optical integrator and a final image on a mask; while themask moves along a scan direction, changing the light pattern on theobject area so that a length of the final image along the scan directiongradually increases at the beginning of a scan cycle and graduallydecreases at the end of the scan cycle.